The patterns of evolution in groups of free-living and parasitic unicellular protists are poorly understood at the population, species and generic levels. Modern molecular and morphometric methods have rarely been applied to these forms, and consequently, their patterns of evolution cannot be inferred with accuracy. The small amount of modern data that do exist indicate that models of the patterns of evolution inferred from research on higher multicellular forms probably cannot be applied to unicellular forms. The progress of modern research on natural populations and species of protists has been slow due to the lack of appropriate "model organisms" on which research efforts from different labs can be focused. The marine diatom, Skeletonema costatum, is proposed as a model system for the study of free-living photosynthetic protists in the chromophyte lineage. Past work conducted in this laboratory has already established that populations of this organism are as genetically different at the molecular level as different genera of higher plants, but are not significantly different at the morphological level. Patterns of differentiation in chloroplast DNA (cpDNA) are currently being examined. This is the first quantitative study of cpDNA variation at the population level in any protist. This research is currently being extended to nuclear genes, including genes and spacer regions in the rDNA cistron and a single-copy nuclear sequence. Arbitrarily primed PCR is also being used to develop new probes. The concordance of the patterns of genetic diversity inferred from molecular differences are being compared to patterns of cell morphology obtained from scanning electron micrographs analyzed morphometrically by a computer-integrated digital imaging system. The relevance of this work to biomedical research is that the organism is being used as a model system to study protist population genetics at the molecular level and many of the findings have broad applicability. Students are involved in all phases of the work and are closely supervised directly by the P.I.